4 Answers
4

Pdfs are kinda human readable

Barring security passwords, much of it is human readable. If a PDF has a password, all the strings and streams (which will already be compressed, no loss) will be pseudorandom garbage. Compressed data streams abound, but much of it looks something like this in your favorite text editor:

Indirect References point to other objects in the PDF:
< objNum > < generationNum-AlwaysZero > R

In the above example object, the content stream is in object 4, elsewhere in the PDF. To find it, you can use your editors text search for "N 0 obj" where N is the object number you want.
WARNING: There are hundreds, possibly thousands of objects in a PDF. Searching for "1 0 obj" will get you a LOT of hits.

Given that you're asking to see the internal structure, you probably already know all this. Others wanting to know the same thing may not.

WARNING: Do not EDIT a PDF in a text editor. All that binary stuff will get mangled, byte offsets are Very Important in PDF.

Acrobat Plugin[s]

There's an acrobat plugin called PDF CanOpener by Windjack Solutions (no affiliation). It's SLICK. You'll be able to browse the PDF structure as a tree, look at (and modify) content streams, and so forth.

Thirdy Party Apps

Lots. Many folks build one as part of learning to parse PDF, or as a debugging tool. They're Quite Handy.